1. Field of the Invention
The present invention relates to a wind turbine provided with a nacelle, in which dentation such as serration formed of triangular or trapezoidal teeth is formed in the trailing edge part of each of the blades of the turbine along the length of the blade.
2. Description of the Related Art
Up to now, wind turbine generator equipment having a large electric power generation capacity have been established at high elevations such as on hills and mountains or on the sea where high wind speed can be utilized, the equipment comprising a number of large wind turbine generating units, each wind turbine being provided with a nacelle and generating rotating force by the wind force acting on a plurality of blades attached to the rotor to drive an electric generator connected to the rotor.
An example of a horizontal-axis wind turbine provided with a nacelle will be explained with reference to FIGS. 12(A) and 12(B). As shown in the drawings, the wind turbine is mounted on the top of a tapered tube-like tower (support) 4 made of, for example, steel or concrete for revolution in a horizontal plane via a nacelle (box for accommodating apparatuses) 5, the horizontal drive shaft 3 of a rotor 2 having three blades 1 in front of the nacelle 5 is supported by the nacelle 5. The horizontal drive shaft 3 is connected to an electric generator(not shown in the drawing) via a speed increasing gearbox (not shown in the drawing) in the nacelle 5.
The diameter of the surface of revolution of the blades of a conventional large horizontal-axis wind turbine reaches nearly 45 m, and the wind turbine to which the present invention is applied can be composed to have the diameter of the surface of revolution of blade of 10 m to 100 m, not limited to nearly 45 m.
FIGS. 13(A), 13(B) and 13(C) show an example of the blade of prior art of a conventional wind turbine provided with a nacelle, when FIG. 13(A) is a partial perspective view, FIG. 13(B) is a sectional view, and FIG. 13(C) is an enlarged partially plan view of the trailing edge part of the blade.
Referring to FIG. 13(A), reference numeral 1 is a blade, 11 is the leading edge part, 12 is the trailing edge part, 1a is the upper chord surface, and 1b is the lower chord surface of the blade 1. As shown in FIG. 13(B), the sectional profile of the blade of the wind turbine is a streamlined one, a profile so-called an airfoil, which is tapered off to the rear edge (trailing edge) and less subjected to air resistance, and rotating force results from the lift F effected through the pressure difference arising from the difference between the velocities of air stream S on the lower chord surface 1b and upper chord surface 1a of the blade 1.
The blade 1 is made of FRP (fiber reinforced plastic) into a one-piece structure or may be made of porous resin such as foamed polypropylene (polybrene) in the case of a small size blade.
A wind turbine blade consisting of a main blade body part and a rear end part is proposed in Japanese Patent Application Publication 2000-120524, in which the rear end part is formed as a separate member and fixed to the main blade body, and the rear end member constituting the rear end part has a plurality of protrusions of triangular or sawtooth-like tooth formed along the length of the blade in its trailing edge part and extending toward the rear.
According to the disclosure, become the rear end member of the wind turbine blade is formed as a separate member and fixed to the main blade body, the rear end member can be worked separately, and the working thereof becomes easy as the handling of the whole of the large blade is not necessary.
The present invention is directed to forming the rear end member with such high precision that the thickness of the trailing edge can be reduced to a value such that the generation of Karman vortex street is suppressed, and further that vertices which interfere with the generation of the Karman vortex street are generated as a result of the protrusions of triangular or sawtooth-like teeth formed in the trailing edge part along the length of the trailing edge part of the separate rear end member and extending toward the rear, and, as a result, noise level is reduced.
By providing the protrusions of triangular or sawtooth-like teeth in the trailing edge part, the generation of Karman vortex street is suppressed compared with the case in which the trailing edge is straight.
Twisting moment about the longitudinal axis of the blade and bending moment act on the wind turbine blade as a result of its own weight and wind force as it rotates, and shearing and tensile stresses as a result of the moments are easy to concentrate in the leading edge and trailing edge. Particularly, the trailing edge part is liable to be fractured by stress concentration as the thickness thereof is necessary to be thin.
According to the present invention, the front end part and the rear end part of the blade are composed of a plurality of separate members respectively and each separate member can be deformed independently. With this composition, the concentration of stress is alleviated and fracture resulting from stress concentration is prevented.
In this prior art, the rear end part of the wind turbine blade having dentation of triangular or trapezoidal teeth formed in the trailing edge part is formed into a separate member or a plurality of separate members made of hard rubber which is tenacious than the FRP of the main blade body and fixed to the rear end of the main blade body.
When the thickness t of trailing edge part 12 is larger than a certain value, Karman vortex street 19 consisting of a couple of vortex sheets is developed downstream from the trailing edge, the vortex being generated periodically with a cycle proportional to wind speed, and the couple of vortex streets A, A1 proceed downstream to B, B1 as shown in FIG. 13(A). Noise with a frequency proportional to wind speed is generated caused by this phenomenon.
In the case of the blade 1 made of FRP shown in FIG. 13, the thickness t of trailing edge of the blade 1 is inevitably relatively large because the sheet of FRP joins at the trailing edge, and it is difficult to suppress the generation of the Karman vortex street 19 completely.
Even when the rear end part (rear end member or members) of the split-type wind turbine blade is made of hard rubber, the thickness t of trailing edge becomes relatively large and as a result the suppression of the noise due to the generation of said Karman vortex street is difficult, so that, in addition to making the rear end part of hard rubber, said dentation is formed in the trailing edge part 12 to allow vertices rotating about the lines parallel to the wind stream passing the tips and roots of the teeth of the dentation to be generated. These vertices suppress the generation of Karman vortex street, and as a result the noise resulting from Karman vortex street is suppressed.
In the prior art as mentioned above, because the main blade body composing most of the blade is made of FRP and the rear end part made of hard rubber is fixed to the rear end of the main blade body, there are problems to be solved such that it is necessary to provide joining part of the FRP sheet in the vicinity of the trailing edge, and that the blade must inevitably be made large in thickness in consideration of the strength of the blade as a whole, which limits blade performance and suppresses improvement of efficiency of the wind turbine.
When the wind turbine provided with a nacelle is in operation, a blade surface boundary layer 21 is formed between air stream S and the surface of the blade 1, i.e., the upper chord surface 1a and lower chord surface 1b as shown in FIG. 13(B) as a result of the construction that an FRP sheet surrounds the vicinity of the trailing edge part. The blade surface boundary layer 21 influences largely upon the performance of the blade 1. When the thickness xcex4 of the boundary layer 21 increases, the flow loss due to the development of the boundary layer increases and blade performance is decreased.
The blade surface boundary layer is a thin layer of air developed on the surface of blade due to the viscosity of air, a thin layer from the surface (where the velocity of air relative to the blade surface is zero) to the place where the velocity of air relative to the blade surface is the same or near the relative velocity of the air stream S outside the boundary layer.
The thickness xcex4 of blade surface boundary layer 21 is given by following Eq (1):
xcex4=cxc2x7Lxc2x7(1/Re)/5xe2x80x83xe2x80x83(1),
where c is a coefficient and its value is about 0.37, L is chord length of the blade, and Re is Reynolds number.
As recognized from above Eq (1), the thickness xcex4 of boundary layer is proportional to the chord length L.
The thickness xcex4 of the boundary layer 21 decreases with decreasing chord length L, and blade performance is increased by decreasing chord length L. Therefore, with the blade of the prior art made of FRP in which an FRP sheet surrounds the vicinity of the trailing edge part, there is a limit to the decrease of the thickness xcex4 of the boundary layer 21.
Therefore, it is desired to provide the blade 1 which allows the formation of the blade surface boundary layer 21 of thickness xcex4 with which good blade performance can be attained while taking into consideration the strength and output (lift) of the blade 1.
As mentioned before, Karman vortex street 19 is generated and proceeds downstream from the trailing edge 12 as shown in FIG. 13(A), and the noise of frequency proportional to the wind speed is generated in the rear near the trailing edge 12. However, with the blade 1 having rear end member made of hard rubber fixed to the rear end of the main blade body as disclosed in Japanese Patent Application Publication 2000-120524, the strength of the trailing edge part 12 is low compared with that of the main blade body, and it is difficult to provide such a blade which is capable of suppressing the generation of said Karman vortex street 19.
The present invention was made in light of the problems mentioned above, and an object of the invention is to provide a wind turbine provided with a nacelle, which has blades strong enough for sporadic heavy load, is highly durable and reliable, is capable of being increased in performance by making it possible to reduce blade thickness, and can suppress the generation of the noise due to Karman vortex street.
Another object of the present invention is to provide a wind turbine provided with a nacelle, in which each blade is formed so that the thickness of the boundary layer generated on the chord surface becomes a thickness so as to be able to attain good blade performance.
The present invention was made to attain these objects, and proposes a wind turbine provided with a nacelle mounted on the top end of a support for horizontal revolution, a rotor having a plurality of blades in front of the nacelle being supported for rotation by the nacelle, a rotating force resulting from the lift effected through the pressure difference arising from the difference between the velocities of air stream on the lower chord surface and upper chord surface of the blades, wherein each of blades is a one-piece blade made of one kind of metal or the rear end part of the blade is made of another metal, and the trailing edge part of the blade is formed to have dentation along the length of the blade.
It is suitable that, when the blade is formed of one kind of metal into a one-piece blade, the one-piece blade is made of light metal such as aluminum, titanium, or aluminum alloy including titanium, and when the blade is of a split-type, the main part (main blade body) of the blade excluding the rear end part of the blade is made of one kind of light metal, and the rear end part is made of another metal having higher strength than the metal of the main blade body, e.g., said main blade body is made of aluminum or aluminum alloy, and said rear end part is made of titanium or steel.
Further, when the blade is of a split-type, the main blade body is formed into a hollow one, the rear end part is formed into a solid one, and both are connected by means of riveting or welding.
According to the invention, the blade can be formed into a thin blade as a whole while maintaining the strength of the blade, because the one-piece blade or the main blade body of the split-type blade is made of high-strength metal, preferably aluminum alloy including high-strength aluminum.
Therefore, blade performance can be largely increased resulting in high efficiency of the wind turbine.
The trailing edge of the blade can be reduced to about 2 mm. In addition to this, the generation of Karman vortex street can be completely prevented by forming dentation in the trailing edge part of the blade along the length of the blade, and the noise resulting from the generation of Karman vortex street is positively suppressed.
Further, even if the blade is formed into a thin blade as a whole, the strength of the blade is secured, for the rear end part of the blade is made of high-strength metal such as titanium or steel. Therefore, a thin, lightweight blade which has enough strength for sporadic heavy load which may act on the blade when a gust of wind or earthquake occurs, can be obtained.
Therefore, a blade having a high durability and reliability is provided such that damage of the blade when such a heavy load acts on the blade is evaded.
Further, because the one-piece blade or the main blade body is made of high-strength light metal, a required blade strength can be secured even when the blade or main blade body is made of one kind of metal, and the provision of core members in the blade for reinforcement is not necessary, as is in the case of the conventional blade made of FRP. Accordingly, the structure of the blade 1 is simplified. The blade 1 can be easily manufactured by press working or casting of aluminum alloy, and production man-hours are largely reduced.
Further, as the main blade body and rear end part (rear end member) are made of metals different from each other, it is possible to form the main blade body, which constitutes most of the weight of the blade, out of light metal such as aluminum alloy including aluminum or titanium alloy including titanium, and the rear end member, which constitutes a relatively small percentage of the weight of the blade, out of high-strength steel or titanium alloy which is easy to be formed into thin trailing edge part and also easy to be formed to have the dentation in the trailing edge part. By this, a wind turbine blade can be obtained which can achieve increased blade performance and positively suppress noise resulting from the generation of Karman vortex street.
It is suitable that the blade according to the present invention has a dentation formed in the trailing edge part along the length of the blade, the dentation being formed in a shape of a serration of triangular teeth, of trapezoidal teeth, or of sawteeth, and the ratio of the tooth height h to the thickness xcex4 of the boundary layer developed on the surface of the blade is:
h/xcex4=1.0xcx9c10.0,
preferably, h/xcex4=2.0xcx9c8.0, and most preferably, h/xcex4=4.0xcx9c6.0.
The larger the ratio h/xcex4 is, the better the effect of noise reduction. However, the values of ratio h/xcex4 shown above are suitable in view of the meaningful effect on the airfoil characteristic and the strength of the tooth in consideration of the height-and-thickness relation thereof.
It is suitable that the tooth height h decreases toward the outer end of the blade along the length hereof so that said ratio (h/xcex4) of the tooth height h to the thickness xcex4 of the boundary layer developed on the surface of the blade is constant along the length of the blade.
It is preferable that the tooth height h is constant along the length of the blade and the chord length L of the blade is decreased toward the outer end of the blade along the length thereof so that the ratio (h/xcex4) of the tooth height h to the thickness xcex4 of the boundary layer developed on the surface of the blade increases toward the end of the blade along the length thereof.
According to the present invention, the flow loss resulting from the blade surface boundary layer in the vicinity of the dentation is decreased and the performance of the blade is increased by increasing the ratio (h/xcex4) of the tooth height h of the dentation to the thickness xcex4 of the blade surface boundary layer, that is, by increasing the tooth height h of the dentation or by decreasing the thickness xcex4 of the boundary layer.
However, when the height h of the dentation is increased, the chord length L of the blade must inevitably be increased, so that the blade becomes larger and the thickness xcex4 of the boundary layer rather increases resulting from the increase of the chord length L.
Although the thickness xcex4 of the boundary layer decreases when the chord length L is decreased, the strength of the blade decreases and the output (lift F) of the blade also decreases with decreased chord length.
Therefore, according to the present invention, the blade is composed so that the ratio (h/xcex4) of the tooth height h of the dentation to the thickness xcex4 of the boundary layer is 1.0xcx9c10.0.
When said ratio (h/xcex4) is smaller than 1.0, the thickness xcex4 of the boundary layer is large and the flow loss resulting from the boundary layer increases and the performance of the blade decreases. On the other hand, when the ratio (h/xcex4) is over 10.0, the tooth height h is high and the chord length L must inevitably be increased. As a result, the width of the blade becomes larger, the thickness xcex4 of the boundary layer rather increases with increased chord length L, and improvement is difficult to be obtained concerning the performance of the blade.
According to the present invention, it becomes possible to attain required blade performance with required blade strength and blade output(lift) secured by composing the blade so that the ratio h/xcex4 is 1.0xcx9c10.0.
Further, according to the present invention, because the tooth height h is decreased toward the outer end of the blade along the length thereof so that the ratio (h/xcex4) of the tooth height h of the dentation to the thickness xcex4 of the boundary layer is constant along the length of the blade, the flow loss resulting from the blade surface boundary layer can be suppressed to a constant value along the length of the blade, and the tooth height h can be decreased in proportion to the chord length L along the length of the blade, resulting in a balanced blade strength and output (lift) along the length of the blade.
Still further, according to the present invention, because the tooth height h of the dentation is constant along the length of the blade and the chord length L of the blade is decreased toward the outer end of the blade along the length thereof so that the ratio (h/xcex4) of the tooth height h to the thickness xcex4 of the blade surface boundary layer increases toward the outer end of the blade along the length thereof, the flow loss resulting from the blade surface boundary layer is decreased toward the outer end of the blade, where the rotating force obtained from wind force is larger for increased radius from the center of rotation axis of blade, and the blade can achieve high efficiency over the length of the blade with the chord length L of the blade determined to the minimum value for securing the strength of the blade.
According to the present invention, the blade is configured such that the dentation of the trailing edge part along the length of the blade is shaped in a serration of triangular teeth, of trapezoidal teeth, or of sawteeth, and the ratio (h/p) of the tooth height h to the pitch p is 0.5xcx9c5.0.
When the ratio (h/p) of the height h to pitch p of the tooth of the dentation is smaller than 0.5, the tooth height h must be small, the trailing edge of the rear end part approximates a flat trailing edge, and the generation of the longitudinal vortex tubes resulting from the dentation and the consequent effect of suppressing the generation of Karman vortex street cannot be expected.
On the other hand, when the ratio h/p is larger than 5.0, the tooth inevitably becomes slim one with narrow width of root, resulting in decreased strength of the dentention.
Therefore, by shaping the dentation such that the ratio h/p is 0.5xcx9c5.0, it becomes possible to suppress the generation of Karman vortex street and positively reduce noise while securing sufficient strength of the dentation.
The ratio h/p is a parameter indicating the degree of protrusion of the tooth of the dentation, which exerts an influence upon the direction of the vortex lines starting from the dentation, and said range of the ratio h/p is most suitable for attaining noise reduction.
The blade as mentioned above is suitable for applying to a large wind turbine provided with a nacelle of which the diameter of the surface of revolution of blades exceeds 10 m and reaches 100 m.